Heretofore, generally, an anion exchange resin is preferably used for removing anions, and as anion exchange resins, various resins are known which have, as functional groups, various anion exchange moieties, for example, amine-type substituent such as primary, secondary and tertiary amino groups and quaternary ammonium. Among them, anion exchange resins of trialkylbenzyl ammonium salt type (e.g. trimethylbenzyl ammonium salt) wherein trialkyl amine is bonded by a covalent bond to the benzyl group of a unit derived from monovinyl aromatic compound, are particularly excellent in the ion exchange capacity and are widely used.
As well known, various hot water and normal temperature water in a power plant are required to be demineralized and purified. Examples of a nuclear reactor used in an atomic power plant include a boiling water reactor (BWR) and a pressurized water reactor (PWR). The former is a reactor where a cooling water is heated and converted into vapor which is then directly supplied to a turbine, and the latter is a reactor where a primary cooling water is heated and supplied to a vapor-generator where a secondary cooling water is heated and converted into vapor which is then supplied to a turbine.
In any type of the above mentioned nuclear reactors, a condensate demineralizer of a reactor water charged with an ion exchange resin is provided in the circulation system of the cooling water to remove radioactive material from the reactor water and to purify the reactor water.
In addition to the above device, the reactor water of hot water or water of a rising temperature is treated with ion exchange resin also in a demineralizing tower for removing boric acid, a boric acid-recovering device, a used fuel pool water-purifying device and/or a condensate demineralizer. The condensate demineralizer is used also in a large once-through boiler in a thermal electric power plant.
Examples of the demineralizer include a device using a mixed bed comprising an anion exchange resin and a cation exchange resin, a device prepared by connecting a single bed of an anion exchange resin with a single bed of a cation exchange resin, and a device prepared by precoating a powdery or fiber-like resin on a nylon or stainless made element.
Among the conventional ion exchange resins, generally, an H type cation exchanger has a satisfactory heat resistance at 120.degree. C., but an OH type anion exchange resin has a heat resistance at 40.degree.-60.degree. C. and a Cl type anion exchange resin has a heat resistance at about 60.degree. C. Thus, in a demineralizer using these conventional anion exchange resins, the performances of the anion exchange resins are degraded and their demineralizing performances are lowered when hot water of 60.degree. C. or higher is passed for a short or long period of time. Therefore, in the treatment of various hot waters in a power plant, a hot water having a temperature of at least 60.degree. C. must be cooled to a water having a temperature of less than 60.degree. C. (generally 50.degree. C. or lower).
Under such a high temperature condition of above 60.degree. C., the anion exchange resins of trimethylbenzyl ammonium salt type undergo detachment of the quaternary ammonium ion exchange groups or the quaternary ammonium group is converted to a tertiary amine group, and they are not suitable for use under a high temperature condition for a long period of time. U.S. Pat. No. 4,025,467 discloses that a strong basic anion exchanger can be obtained by reacting a quaternary amine to a crosslinked copolymer of an aromatic monovinyl hydrocarbon with an aromatic divinyl hydrocarbon having haloalkyl groups of the formula --C.sub.n H.sub.2n --X wherein --X is chlorine or bromine, and --C.sub.n H.sub.2n is an alkylene group with n being from 1 to 4. However, specifically disclosed is only the strong basic anion exchanger wherein n is 1, and there is no description to distinguish between the linear case and the branched case among --C.sub.n H.sub.2n groups. If the one wherein n is 3 or 4 were produced in accordance with the method concretely disclosed in this publication, the resulting --(CH.sub.2).sub.3 --or --(CH.sub.2).sub.4 --would be present only at a part of anion exchange groups of the entire resin, whereby the heat durability would not be improved. However, this publication describes nothing about the heat durability of the resin.
EP-A-400,848 discloses an anion exchanger having ##STR3## unit which is structurally similar to the anion exchange resin used in the present invention, but it is used as an anti-high cholesterol agent, thus its use being essentially different from that of the present invention.
Accordingly, an anion exchanger having higher heat durability has been desired.